When shopping for a compound microscope, it is best to purchase a new scope rather than a used one. Used ones often have been abused by students and no longer work optimally. Key features to look for are a metal construction, achromatic glass lenses, and an easy-to-find light bulb. Binocular and monocular options are available, and the choice should be based on comfort and frequency of use. A binocular compound microscope will cost more than a monocular scope, so the best compound microscope for you will be the one that fits your needs and budget.
The lenses, eyepieces and basic construction of a microscope are the key points to consider when choosing the best compound microscope. The most basic of features relate to the microscope’s construction. The frame should be made from metal, coated with a reagent-resistant finish, and sturdy. The knobs and focus gears should be metal with ball bearings around all moving parts. Toy microscopes that are made from plastic, have plastic lenses, and produce fuzzy images should be avoided.
Standard objective lenses should follow the Deutsche Industrie Norm (DIN) standard for threading and length, and they should be achromatic, glass lenses. If the lens is not achromatic, then it is not color-corrected and you will not be able to view everything through the lens of your scope. Certain types of achromatic lenses, such as the plan or semi-plan lens, are more costly than the standard achromatic lens, which is well suited for student and hobby use.
Most compound microscopes typically include from three to five objectives with a magnification power ranging from 4x to 2,000x. In general, a 400x magnification is suitable for biology work while a 1,000x magnification is better for microbiology samples. Resolution increases with increasing magnification power, which comes from the objective lenses. Increasing the magnification of the eyepiece does not increase resolution.
Some microscope options will include changeable objectives and eyepieces. Changing the eyepiece and objective lenses may be an option for a professional user. Otherwise, the standard wide-field eyepiece with an 18-millimeter (mm) lens is recommended.
In terms of a compound light microscope, the best advice traditionally has been to select a scope that uses a fairly standard and easily obtainable bulb, in case you should need a replacement. Tungsten, or incandescent, bulbs are the most economical but not necessarily the optimal choice. Fluorescent bulbs are better than tungsten because they produce a whiter light and truer colors; these probably are the best option for student and hobby use. Halogen and light-emitting-diode (LED) bulbs typically are used on professional-grade scopes, which cost more money. If you want dimmer capabilities, then the halogen and LED options will have a dimmer switch.
The ability to focus on detailed images under magnification is the ultimate goal when using a microscope. It follows that the best compound microscope will have a fine adjustment knob in addition to the coarse adjustment knob. The fine focus may be extra but, without it, you may not be able to see tiny details such as an ant’s eye or cellular structures.